Biology OCR As Level F3211 Cell Membranes

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  • Created by: meg
  • Created on: 14-05-13 10:57
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  • Cell Membranes
    • Every cell is surrounded by a selectivley permeable plasma membrane, which controls what passes through it. The plasma membrane also has an important role in cell signalling.
    • Many organelles are also surrounded by membranes; these membranes help to isolate the metabolic reactions inside the organelle from those outside it, and provide extra surface area for the attachment of enzymes and other molecules.
    • Membranes are made of a phospholipid bilayer in which proteins are embedded. This is known as the fluid mosaic model. The membranes also contain cholesterol, glycolipids & glycoproteins.
    • Cells are able to send and receive signals e.g. in the form of molecules such as hormones. Such signals are recieved by the plasma membrane; the arrival of a signal may bring about a response in the cell
    • Substances that have small, uncharged molecules can diffuse passively through the phospholipid bilayer. Larger molecules and charged ions pass through chanels formed by proteins. If they are diffusing passively down their concentration gradient, this is known as facilitated diffusion.
    • Water molecules can move freely across most membranes, by diffusion, down their water potential gradient. This is osmosis
    • Cells placed in a solution that has a lower water potential than the cell contents lose water by osmosis, so their volume decreases. Animal cells may become crenated whilst the cell membrane in plant cells may pull away from the cell wall as the cytoplasm shrinks.
    • Cells placed in a solution that has a higher water potential than the cells contents gain water by osmosis, so their volume increases. Animal cells may burst, but plant cells do not because of the strong cell wall that surrounds them
    • Substances can also be moved across membraes against their concentration gradient, using energy in the form of ATP produced by the cell. This is active transport & it takes place through carrier proteins in the membrane
    • Substances can be moved across a membrane in bulk by exocytosis or endocytosis.
    • An increase in temperature increases the movement of the molecules in a membrane, increasing the membranes permeability.


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